The Involvement of the Brain's Opioid System in Runner's High

Multiple Choice

The involvement of the brain's opioid system in runner's high suggests one possible explanation for why:

A) endurance athletes enter a meditative state during long training sessions.
B) sedentary people find it so difficult to begin an exercise routine.
C) some people can become addicted to excessive exercise.
D) athletes perform better when cheered on by a crowd.